How can space travel be like world travel?

[Bullgrit]

If I put two dots on a balloon, and then measure the speed of light between them, and then put more air in the balloon so that the dots are farther apart, and do the same measurement again, would you assume that light was now travelling slower?

I don't see how this question makes any sense.

I was asking how can we see light from something that is further away in lightyears than the universe has existed in years.

Bullgrit

 

[Umbran]

I was asking how can we see light from something that is further away in lightyears than the universe has existed in years.

The answer is that we don't. Not really.

14 billion years ago, a thing that was 14 billion light years away sent out light. We are receiving that light now.

But the thing isn't sitting still for those 14 billion years. It moved (or, you might say, space got added between us and it). We know something about that motion, and we list what its distance from us would be now, not what it was back then.

Now, the balloon analogy has some relevance, but not as simply as jonsey put it.

Imagine a balloon, not inflated. Put two dots on the balloon, and start to inflate it. Of course the distance between the dots increases, they appear to move, even though each one thinks it is sitting still on its spot on the balloon.

Now, how does this get us to that "faster than light" thing? Here's how:

We humans generally measure distances along the surface of the balloon, and we assume it is a flat sheet that might stretch, but always remains flat. If you measure that distance, yes, it can look like the objects are moving faster than light.

But, the balloon's surface is curved. The real spacetime distance between the objects is measured through the balloon, kind of like you stuck a straight, thin wire through both dots. Along this line, the distance is shorter than along the surface of the balloon, and the distance between them grows more slowly as the balloon inflates, so that they never move faster than light.

 

[Bullgrit]

OK. But...

14 billion years ago, a thing that was 14 billion light years away sent out light. We are receiving that light now.

Shouldn't we be seeing a single point, (the Big Bang)? But then, 14 billion years ago, weren't "we" right there in the same spot as the BB? I understand the idea that we see things where they were when they shed the light we see now, and that they have moved since.

If we can see something 15 billion lightyears away, that means it's light left it 15 billion years ago. But the universe didn't exist 15 billion years ago. That seems sort of like finding fossils on Earth that date back 5 billion years.

For the record, (in case I need to say it): I'm not saying anything like "science is wrong"; I'm just asking to get an understanding of why/how these things work.

Bullgrit

 

[Umbran]

OK. But...
Shouldn't we be seeing a single point, (the Big Bang)? But then, 14 billion years ago, weren't "we" right there in the same spot as the BB?

Okay, a couple of things. First off, we often use approximate numbers. Technically, the best accepted estimate is that the Universe is 13.75 (+/- some) years old. So, when I say 14 billion, I'm rounding off for simplicity.

Next: The "visible universe" is what we can *theoretically* see, not what we've actually gotten a picture of today. Nobody has actually ever imaged the Big Bang. If I recall correctly, the most distant object ever detected was a gamma ray burst from a dying star that was some 13.14 billion light years away when it went boom (the explosion thus took place 13.14 billion years ago).

It is estimated that the first stars formed some 200 million years after the Big Bang, so 13.55 billion years ago. The camera to see those things hasn't been built yet. We only saw that gamma ray burst because it was a huge burst, incredibly bright. We don't see steadily burning stars that far away, yet. But, by that time the Universe was already big enough that there were places billions of light years away for stars to be created in.

Before that (or farther away, depending how you want to look at it), there's probably no objects to image at all. Nothing to see but a haze of glowing gas. Go even farther back, and you reach a time when photons haven't been invented yet, so you cannot catch them to "see" anything at all.

 

[BrooklynKnight]

gas. Go even farther back, and you reach a time when photons haven't been invented yet, so you cannot catch them to "see" anything at all.

I know i'm nitpicking but I can't help it. Invented implies intelligence. Don't you mean "came into existence" or "formed" or whatever.

 

[Umbran]

I know i'm nitpicking but I can't help it.

I'm pretty sure you could have helped it. You chose not to.

Ironic, then, you using technically incorrect language to call me out on use of technically incorrect language.

Invented implies intelligence. Don't you mean "came into existence" or "formed" or whatever.

I think the semantic argument about whether invention really requires intelligence (noting that we are in an era with self-modifying computer code) is a philosophical topic for another thread. I think the argument over whether the Universe can invent things is nigh on to religion, and thus not appropriate for these forums. And, I think what I meant is still perfectly clear in context.

Do you want to continue this nitpick war, or shall we return to the subject of the thread?

 

[jonesy]

If we can see something 15 billion lightyears away, that means it's light left it 15 billion years ago.

No, because we aren't seeing something 15 billion light years away.

What we are seeing is light from which we can glean information of a location. But that location is no longer where information we glean from that light suggests it is. The location is moving, the space between it and us is expanding, and all space has been expanding the entire time that the light has been moving.

Also, we came late to party even when the party was late. Imagine all the information contained in the light which has passed us in the time it has taken us to become us.

 

[Joker]

I'm pretty sure you could have helped it. You chose not to.

Ironic, then, you using technically incorrect language to call me out on use of technically incorrect language.



I think the semantic argument about whether invention really requires intelligence (noting that we are in an era with self-modifying computer code) is a philosophical topic for another thread. I think the argument over whether the Universe can invent things is nigh on to religion, and thus not appropriate for these forums. And, I think what I meant is still perfectly clear in context.

Do you want to continue this nitpick war, or shall we return to the subject of the thread?

Woah, where's a 'katty' emote when you need one ?

 

[Pbartender]

No, because we aren't seeing something 15 billion light years away.

What we are seeing is light from which we can glean information of a location. But that location is no longer where information we glean from that light suggests it is. The location is moving, the space between it and us is expanding, and all space has been expanding the entire time that the light has been moving.

Also, we came late to party even when the party was late. Imagine all the information contained in the light which has passed us in the time it has taken us to become us.

Or to put it another way, you'd be looking at something that used to be 15 billion light years away, 15 billion years ago.

 

[Bullgrit]

No, because we aren't seeing something 15 billion light years away.

I think you're being a bit too persnickety, here. You know what I meant, and you're not addressing what my question/point. How can something have existed, (however we detect it, now), 15 billion years ago, (or 15 billion lightyears away)? Existence didn't exist 15 billion years ago.

Unless the universe is/was at some time expanding at a rate faster than lightspeed?

Bullgrit